Mutations are any event that changes the payable balance on a contract. You can find these by clicking on the Balance tab for any contract.
Recoupable advances, statement payouts, and any other recoupable costs should be added to the contract balance sheet as a negative mutation, reducing the contract's payable balance. When a statement is closed, the Amount Due will be automatically added to the relevant contract as a positive mutation (thereby increasing the contract's payable balance). Non-recoupable payments should not be added as mutations.
Mutations can be used to zero-out a contract's balance if a statement has been paid out. For example, if a statement added $500 to the contract's balance, a mutation of -$500 would indicate that the balance was paid out and bring the contract balance back to zero. This can also be done automatically: for any contract with a positive balance, clicking the Zero Out button will automatically create a negative mutation to bring the balance back to zero. Clicking the Zero Out button on a statement run will do this for all contracts with a positive balance in the run, above a user-set threshold.
Mutations can also be used to correct accounting errors, similarly to an adjustment. If a prior statement over- or under-reported royalties, a subsequent mutation for a positive or negative amount can be used to fix the contract balance.
Each new statement on a contract will include all mutations added since the last statement was closed. Mutations can be edited and deleted until the statement on which they are included gets closed. Once a mutation is on a closed statement, it becomes locked and can no longer be edited or deleted.